The invention relates to a hand-actuatable pump for feed pumps of fuel injection systems for internal combustion engines. Manually actuatable pumps of this kind serve to fill the system when it is put into operation, or for refilling and venting after some intervention has been made, such as changing a filter insert or filter box. In the position of repose of the pump, the piston has to be locked in the cylinder, to prevent pressure impacts (pressure peaks) of the feed pump from damaging or even destroying the piston or the seals, which would render the pump inoperable.
U.S. Pat. No. 3,803,988 discloses a pump of this kind, in which the locking is effected by means of a bayonet lock for locking the actuating element to the cylinder. Another type of pump is described having a piston with a threaded protrusion and a cylinder having a corresponding internal thread. Locking is effected here by screwing the threaded protrusion into the internal thread.
In practice, however, it has been found that the locking did not always take place, which caused the above-mentioned damage to the pump, or even its failure.